Cooking utensils surface coated with tetrafluoroethlene base polymer

ABSTRACT

A tetrafluoroethylene base polymer-containing coating composition comprising a dispersion of a tetrafluoroethylene base polymer having a molecular weight of at least 5,000,000 in a liquid medium containing a surfactant, which is particularly useful for coating a fixing heater roller and a cooking utensil.

This application is a continuation of application Ser. No. 07/466,780filed on Jan. 18, 1990, now abandoned, which is a divisional ofapplication Ser. No. 07/376,230 filed Jul. 3, 1989, now U.S. Pat. No.4,914,146 which is a continuation of Ser. No. 07/076,020 filed Jul. 21,1987, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polytetrafluoroethylene (PTFE)containing coating composition and the use thereof. More particularly,it relates to a coating composition containing PTFE having a molecularweight of at least 5,000,000 and an article having a surface which iscoated with said PTFE such as a fixing heater roller and a cookingutensil.

2. Description of the Prior Arts

To impart non-tackiness and stain-proofing properties to variousarticles, a fluororesin, particularly PTFE is coated on the surface ofthe articles.

Since PTFE has very poor adhesivity to the article material, variousattempts have been made to improve the adhesivity of PTFE to thematerial by adding additives to the PTFE containing coating composition.However, none of the conventional PTFE containing coating composition issatisfactory.

Particularly, a cooking utensil is used under very severe conditionssuch as a very high temperature and has to withstand rubbings with ametallic or wooden tool or corrosion by seasonings such as salt,vinegar, oil, etc., for an extended period of time.

In an electrostatic process copying machine which is now mainly used asa copying machine or a facsimile machine, a toner image is fixed onto asheet of paper by heating and pressurizing the sheet of paper bearingthe toner image between a fixing heater roller and a pressure roller.

During the fixing of the toner image on the paper, the peripheralsurface of the fixing heater roller is heated at a temperature of 150°to 200° C. so that the toner image to be fixed is melted onto thesurface of the paper. Therefore, the toner image tends to adhere to theperipheral surface of the fixing heater roller. Once the toner adheresto the surface of the fixing heater roller, an offset phenomenon of theimage occurs, and causes the copied image and/or the paper to be dirty.

To prevent the offset phenomenon, the peripheral surface of the fixingheater roller is covered with a fluorine-containing resin having goodreleasing properties and heat resistance such as PTFE (cf. U.S. Pat. No.4,568,275). Since PTFE is negatively charged, it easilyelectrostatically couples with the toner which is generally positivelycharged so that the toner easily adheres to the PTFE. Further, sinceconventionally used PTFE has a molecular weight of not greater thanabout 4,000,000, it is crystalline and has a band structure. Thus, acoating comprising conventional PTFE, tends to have irregularity on itssurface and has poor releasing properties.

Also proposed is a fixing heater roller having a peripheral surfacewhich is coated with a tetrafluoroethylene/perfluoroalkyl vinyl ethercopolymer (hereinafter referred to as "PFA") and practically used.However, such a fixing heater roller covered with PFA, does not havesatisfactory durability or offset resistance.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a tetrafluoroethylenebase polymer-containing coating composition which can form a coating ofthe tetrafluoroethylene base polymer on various materials and has goodheat resistance, wear resistance, scratch resistance and the like.

Another object of the present invention is to provide a fixing heaterroller having good wear resistance, scratch resistance and durability.

A further object of the present invention is to provide a fixing heaterroller by which a non-offset range can be set wider than theconventional fixing heater roller.

A yet another object of the present invention is to provide a cookingutensil having a surface coating of PTFE which does not become flawed orremoved after an extended period of use.

According to one aspect of the present invention, there is provided atetrafluoroethylene base polymer-containing coating compositioncomprising a dispersion of a tetrafluoroethylene base polymer having amolecular weight of at least 5,000,000 in a liquid medium containing asurfactant.

According to another aspect of the present invention, there is provideda fixing heater roller comprising a roller base member and a coveringlayer which comprises a tetrafluoroethylene base polymer having amolecular weight of at least 5,000,000 and which is covered on theperipheral surface of the base member.

According to a further aspect of the present invention, there isprovided a cooking utensil having a surface which is coated by a layerof a tetrafluoroethylene base polymer having a molecular weight of atleast 5,000,000.

In the present invention, the tetrafluoroethylene base polymer mayoptionally contain PFA.

DETAILED DESCRIPTION OF THE INVENTION

The tetrafluoroethylene base polymer is intended to define a homopolymerof tetrafluoroethylene having a molecular weight of at least 5,000,000,preferably not higher than 9,000,000 or a copolymer of the samemolecular weight consisting of tetrafluoroethylene and less than 1% byweight, preferably less than 0.5% by weight of a comonomer as amodifier. Examples of the comonomer are hexafluoropropylene, vinylfluoride, vinylidene fluoride, perfluoroalkyl vinyl ether (e.g.perfluoromethyl vinyl ether, perfluoroethyl vinyl ether andperfluoropropyl vinyl ether), chlorotrifluoroethylene, ethylene,propylene, acrylates, etc. Among the tetrafluoroethylene base polymers,PTFE is preferred. As the tetrafluoroethylene base polymer, one preparedby emulsion polymerization having a primary particle size of 0.1 to 0.5μm is preferably used.

According to a study by the present inventors, when thetetrafluoroethylene base polymer has a molecular weight of 5,000,000 orhigher, its crystallinity decreases. The coating of such a polymer formsless pin holes and has better surface smoothness than a coatingcomprising a crystalline tetrafluoroethylene base polymer having amolecular weight lower than 5,000,000.

In the present specification, the molecular weight is the "numberaverage molecular weight". The molecular weight (Mn) of thetetrafluoroethylene base polymer is calculated by the following equationfrom a specific gravity (hereinafter referred to as "S.G.") of thepolymer:

    log.sub.10 Mn=28.524-9.967×S.G.

According to this equation, for example, the molecular weight of5,000,000 and 8,000,000 correspond to S.G. of 2.19 and 2.17,respectively. In other words, the term "molecular weight of 5,000,000 to8,000,000" used in the present specification has the same meaning as"S.G. of 2.19 to 2.17".

The S.G. of the polymer can be measured in the following manner:

A sample of the polymer powder (5 g) is compressed in a mold having aninner diameter of 32 mm in an atmosphere kept at a temperature of 23° to25° C. under a pressure of 200 kg/cm², and the sample thus treated isremoved from the mold and sintered in an air heating furnace at atemperature of 380° C. for 30 minutes, cooled to 300° C. at a coolingrate of 70° C./hour, and then the sintered sample is removed from thefurnace and allowed to cool to room temperature in air. The weight ofthe sample thus obtained is measured. The S.G. is defined as follows:##EQU1##

The content of the tetrafluoroethylene base polymer in the coatingcomposition is usually from 10 to 60% by weight.

Examples of the surfactant contained in the coating composition arenonionic surfactants (e.g. Nonion HS-208 manufactured by Nippon Oil andFat Co., Ltd.), anionic surfactants (e.g. Emal manufacture by Kao) andthe like. The content of the surfactant in the coating composition isfrom 5 to 10 parts by weight per 100 parts by weight of thetetrafluoroethylene base polymer.

The liquid medium may be any one of the liquid mediums which areconventionally used for the preparation of the coating compositioncontaining PTFE. Preferred examples of the liquid mediums are water andorganic solvents (e.g. toluene, xylene, etc.).

The coating composition of the present invention may be prepared byadding the surfactant and optional solid additives for modifying thecoating properties of the dispersion of the tetrafluoroethylene basepolymer in the liquid medium.

For example, the coating composition may contain mica powder or flakesor mica flakes covered with a pigment. The mica powder or flakepreferably has an average particle size of 10 to 100 μm, particularly 15to 50 μm. When the mica powder or flakes are contained in the polymer,the content of the mica powder or flakes in the polymer is from 1 to 10%by weight, preferably 2 to 8% by weight.

In addition, the coating composition may contain electrically conductiveparticles or fibers, which can be any of the commercially available onessuch as particles or fibers of carbon or metal (e.g. silver andaluminum). The content of the electrically conductive particles orfibers is from 1 to 20% by weight, preferably from 2 to 10% by weightbased on the weight of the polymer.

Further, the coating composition may contain a pigment for coloring thecoated tetrafluoroethylene base polymer (e.g. chromium oxide, titaniumoxide, carbon black, etc.), an antiwear additive (e.g. metal powder,ceramic materials, etc.), an organic polymer for improving theadhesivity and/or surface smoothness of the coated tetrafluoroethylenebase polymer (e.g. melting fluororesins, acrylic resins, andheat-resistant polymers such as polyamide, polyimide,polyallylenesulfide and polyethersulfone).

When the electrically conductive particles or fibers and/or otheradditives are contained together with the mica flakes, their totalamount is preferably not more than 20% by weight based on the weight ofthe polymer.

In addition, the tetrafluoroethylene base polymer optionally contains 5to 30% by weight of PFA. In the present invention, PFA contains 1 to 10%by weight of the perfluoroalkyl vinyl ether. Preferably, it has a meltviscosity of 0.5×10⁴ to 20×10⁴ poise (at 380° C.).

The tetrafluoroethylene base polymer and optionally PFA may be coated onthe peripheral surface of the roller base member by a per seconventional method. For example, after a primer coating is applied onthe peripheral surface of the roller base member, a suspensioncomprising the tetrafluoroethylene base polymer and optionally theadditives and/or PFA is coated on the peripheral surface of the rollerbase member followed by evaporation of the solvent and baking.

The fixing heater roller of the present invention has good heatresistance and scratch resistance, and does not suffer from initialoffset or stain of the roller surface. Thus, it has improved durability.

Further, the fixing heater roller of the present invention has a largernon-offset range and improved copying reliability. For example, thenon-offset range of the fixing heater roller of the present invention iswider by about 10° C. at both of the higher and lower limits than theconventional fixing heater roller.

Since the fixing heater roller of the present invention has goodnon-adhesivity, a cleaning system of the copying machine can be madesimple.

The cooking utensil may be any one of the conventional cooking utensilssuch as a frying pan, a cooking plate, a grill pan, an egg baking panand other pans.

The present invention will be hereinafter explained further in detail byfollowing examples, in which parts and % are by weight unless otherwiseindicated.

EXAMPLE 1

A roller base member made of aluminum having an outer diameter of 40 mmand a length of 340 mm is used. After the usual surface preparation ofthe roller base member for fluorine-containing resin coating (blasttreatment and primer coating), a coating composition consisting of anaqueous dispersion containing PTFE (Mn=6 million) and mica flake(average particle size=35 μm) in a weight ratio of 97:3 is applied toproduce a fixing heater roller having a coating of about 30 μm inthickness.

EXAMPLE 2

In the same manner as in Example 1 but using an aqueous dispersioncontaining PTFE (Mn=6.5 million), the tetrafluoroethylene/perfluoroalkylvinyl ether copolymer (content of fluoroalkyl vinyl ether=3%, meltviscosity=1×10⁴ poise) and conductive carbon powder in a weight ratio of85:10:5, the roller base member is coated to produce a fixing heaterroller.

EXAMPLE 3

In the same manner as in Example 1 but using no mica flake, the rollerbase member is coated to produce a fixing heater roller.

COMPARATIVE EXAMPLE 1

In the same manner as in Example 1 but using PTFE having a molecularweight of 3,000,000, the roller base member is coated to produce afixing heater roller.

COMPARATIVE EXAMPLE 2

On the same aluminum roller base member as used in Example 1, atetrafluoroethylene/perfluoroalkyl vinyl ether copolymer containing nomica flake is powder coated to form a coating having a thickness ofabout 30 μm on the peripheral surface of the roller base member.

EXPERIMENT 1

As a fixing heater roller, each roller produced in Examples 1-3 orComparative Examples 1-2 is installed in a xerography type copyingmachine. Copy is made on a predetermined number of sheets (20,000sheets, 50,000 sheets or 100,000 sheets) of paper at a fixingtemperature of 180° to 200° C. and a copying rate of 180 mm/sec.(without any releasing oil). Thereafter, the maximum wear depth on theroller surface is measured by a tracer method surface roughness meter.The results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                  Maximum wear depth (μm)                                          Example   After copying                                                       No.       20,000 sheets                                                                            50,000 sheets                                                                            100,000 sheets                                ______________________________________                                        1         1-2        1-2        2-5                                           2         1-2        1-2        1-3                                           3         1-2        1-2        4-6                                           Comp. 1   7-9         7-10       9-16                                         Comp. 2   5-8         7-10      10-15                                         ______________________________________                                    

EXPERIMENT 2

As a fixing heater roller, each roller produced in Examples 1-3 orComparative Examples 1-2 is installed in a xerography type copyingmachine. Copy is carried out at a fixing temperature of 180° to 200° C.and a copying rate of 180 mm/sec. under surface pressure of 40 kg. Whenthe roller of Comparative Example is used, the roller surface is stainedafter 1,000 sheets of A-4 sized paper are copies and offset occurred,while when the roller of the present invention of Example is used,adherence of the toner or offset did not occur after copying 20,000sheets of paper.

In the following examples, a frying pan as a typical example of thecooking utensil is coated by the coating composition of the presentinvention. In these examples, coating of the composition on an aluminumfrying pan and evaluation of the properties of the coated layer arecarried out as follows:

Coating of the composition

After treating a surface of an aluminum frying pan having a thickness of2.4 mm, a primer for fluororesin (Polyflon Enamel (trade mark) EK-1909BKN manufactured by Daikin Industries Ltd.) is spray coated on thesurface of the frying pan and dried. Then, a coating composition isspray coated on the primer and dried in an infrared dryer at about 100°C. for 10 minutes. Thereafter, the coated frying pan is baked in anelectric furnace at 380° C. for 10 minutes to form a fluororesin coatinglayer of 30 μm in thickness including the primer. The thus coated fryingpan is subjected to the following tests.

Wear resistance

Before the test, five parallel lines with each distance of 25 mm are cutto a depth reaching the surface of the aluminum base. With fixing thefrying pan, a commercially available metal scrubbing brush (made of 18-8stainless steel wire, 50 g) is placed on the surface of the frying panhaving the cut lines and reciprocated with applying a load of 1 kg onthe average in a direction perpendicular to the direction of the linesat a rate of 70 times/min. with such amplitude that the center of thebrush crosses at least 3 cut lines (namely, an amplitude of at least 50mm). Simultaneously, the brush is rotated at 70 rpm, and the frying panis heated at about 200° to 220° C. After the brush is reciprocated forup to 10,000 times under such conditions, peeling of the coating isobserved. However, when the coating is peeled off between two cut linesand the aluminum base is continuously exposed between the lines before10,000 time reciprocation, the test is stopped.

Cooking test

On the coated aluminum frying pan, two loafs of chinese noodle, slicedvegetables and meat and seasonings are fried for 17 minutes on theaverage on gas flames with medium strength while mixing them with awooden knife to prevent scorching. The cooking of the noodle isrepeated, and sticking of the noodle to the surface of the frying pan isobserved. The number of cooking cycles is recorded until the scorchingis not removable by the wooden knife or the scorching mark remains.

EXAMPLE 4

To a 60% aqueous dispersion of PTFE (Mn=6 million) (8,000 parts)contained in a stirring tank having a propeller stirrer, a 25% aqueoussolution of sodium lauryl sulfate (195 parts), a 25% aqueous solution ofpolyoxyethylene type nonionic surfactant (1,840 parts), toluene (152parts) and pure water (1,255 parts) are added in this order withstirring. Then, the mixture is stirred for 20 minutes to prepare anaqueous coating composition, which has a solid content of 42%, pH of 9.5and viscosity of 210 cp (at 25° C.).

The properties of the coating formed on the frying pan from thiscomposition are shown in Table 2 below.

EXAMPLE 5

The coating composition prepared in Example 4 (2,380 parts) is chargedin the stirrer. To the composition, mica powder having an averageparticle size of 35 μm (200 parts) and a 24.4% aqueous paste of carbonblack (8 parts) are added in this order with stirring and stirred for 20minutes in total to prepare a coating composition, which has a solidcontent of 42%, pH of 9.5 and viscosity of 220 cp (at 25° C.).

The properties of the coating formed on the frying pan from thiscomposition are shown in Table 2 below.

EXAMPLE 6

In the same manner as in Example 4 but using PTFE having a molecularweight of 5,400,000, a coating composition is prepared.

The properties of the coating formed on the frying pan from thiscomposition are shown in Table 2 below.

EXAMPLE 7

In the same manner as in Example 5 but using the coating compositionprepared in Example 6, a coating composition is prepared.

The properties of the coating formed on the frying pan from thiscomposition are shown in Table 2 below.

EXAMPLE 8

In the same manner as in Example 4 but using PTFE having a molecularweight of 7,100,000, a coating composition is prepared.

The properties of the coating formed on the frying pan from thiscomposition are shown in Table 2 below.

EXAMPLE 9

In the same manner as in Example 5 but using the coating compositionprepared in Example 8, a coating composition is prepared.

The properties of the coating formed on the frying pan from thiscomposition are shown in Table 2 below.

COMPARATIVE EXAMPLE 3

In the same manner as in Example 4 but using PTFE having a molecularweight of 3,000,000, a coating composition is prepared.

The properties of the coating formed on the frying pan from thiscomposition are shown in Table 2 below.

COMPARATIVE EXAMPLE 4

In the same manner as in Example 5 but using the coating compositionprepared in Comparative Example 3, a coating composition is prepared.

The properties of the coating formed on the frying pan from thiscomposition are shown in Table 2 below.

COMPARATIVE EXAMPLE 5

In the same manner as in Example 4 but using PTFE having a molecularweight of 4,500,000, a coating composition is prepared.

The properties of the coating formed on the frying pan from thiscomposition are shown in Table 2 below.

COMPARATIVE EXAMPLE 6

In the same manner as in Example 5 but using the coating compositionprepared in Comparative Example 5, a coating composition is prepared.

The properties of the coating formed on the frying pan from thiscomposition are shown in Table 2 below.

                  TABLE 2                                                         ______________________________________                                        Example   Wear resistance                                                                              Cooking test                                         No.       (No. of reciprocations)                                                                      (No. of cooking)                                     ______________________________________                                        4         6,000          >800                                                 5         >10,000        >800                                                 6         5,000          >800                                                 7         >10,000        >800                                                 8         8,000          >800                                                 9         >10,000        >800                                                 Comp. 3   2,100           230                                                 Comp. 4   3,300           350                                                 Comp. 5   2,900           450                                                 Comp. 6   4,000           550                                                 ______________________________________                                    

What is claimed is:
 1. A cooking utensil, a surface of which is coated with a layer comprising tetrafluoroethylene base polymer having a molecular weight of at least 5,000,000 and an effective amount of not more than 20% by weight based on the weight of the base polymer of an organic polymer for improving the adhesivity and/or surface smoothness.
 2. The cooking utensil according to claim 4, wherein the tetrafluoroethylene base polymer further contains a solid additive for modifying properties of the coating. PG,22
 3. The cooking utensil according to claim 2, wherein the solid additive is at least one selected from the group consisting of mica powder or flakes, a pigment, metal powder, a ceramic material and an antistatic agent.
 4. The cooking utensil according to claim 3, wherein the solid additive is mica powder.
 5. The cooking utensil according to claim 1, wherein the tetrafluoroethylene base polymer is polytetrafluoroethylene.
 6. The cooking utensil according to claim 1, wherein the tetrafluoroethylene base polymer has a molecular weight of 5,000,000 to 9,000,000.
 7. The cooking utensil according to claim 5, wherein the polytetrafluoroethylene has a molecular weight of 5,000,000 to 9,000,000.
 8. The cooking utensil according to claim 1, wherein the organic polymer is a heat-resistant polymer. 